ECG-gated gamma ray energy spectra from the cardiac left ventricle were obtained in 10 subjecs at rest. Nine of the subjects were studied with a non-imaging sodium iodide "probe", with a colimator identical to that used for gamma camera studies. The tenth was studied using an ultra high resolution Germanium detector. Analysis of these data led to the discovery that clinical parameters such as ejection fraction were influenced by the precise energy window setting used. It was found that moving the standard 20% energy window across the photopeak resulted in a 3.4 EF units change in ejection fraction per 10 KeV increase in energy window settings. This finding also has implications for clinical studies performed with asymmetric energy windows. It was found that placing the energy window over the upper half of the photopeak gave an ejection fraction 12% higher than would be obtained with a symmetrically placed window. These results are now being submitted for publication.